In conventional signal transmission/reception, e.g., differential transmission, waveform irregularities such as reflection is prevented by impedance match between transmission paths, as in a signal transmitting/receiving apparatus 1000 shown in FIG. 9A. In order to achieve this impedance match, a receiving device 130 is provided with: a terminating resistor 105 for short circuiting a pair of differential lines 103A and 103C (i.e., data lines); and a bias generating circuit 102 for determining an intermediate potential between differential potentials, where the output of the bias generating circuit 102 is connected at a midpoint of the terminating resistor 105. This will set the intermediate potential of the pair of differential lines 103A and 103C to Vcm, which is a bias voltage output from the bias generating circuit 102, whereby the problem of waveform irregularities such as the reflection between the pair of differential lines 103A and 103C is solved. In the case where the difference between a supply voltage VCC1 of a transmitting device 120 and a supply voltage VCC2 of a receiving device 130 and the difference between a ground voltage GND1 of the transmitting device 120 and a ground voltage GND2 of the receiving device 130 are not large, the intermediate potential between the pair of the differential lines 103A and 103C of a transmitting device 120 is also around Vcm.
The amplitude potential of the pair of differential lines 103A and 103C is determined by a value of a current flowing through the differential lines 103A and 103C, and by a value of the terminating resistor 105. Since the impedance of the differential lines 103A and 103C is usually 110 Ω, the value of the terminating resistor 105 is also set to 110 Ω for impedance matching. Thus, when a driver circuit 101 of the transmitting device 120 applies a 2 mA current to the transmission path 110, the amplitude voltage of the differential lines 103A and 103C will be 220 mV. If the bias potential is 2.0 V, the higher potential of the differential lines 103A and 103C will be 2.11 V (2.0 V+220 mV/2), and the lower potential of the differential lines 103A and 103C will be 1.89 V (2.0 V−220 mV/2).
Therefore, if the driver circuit 101 of the transmitting device 120 applies a stable 2 mA current to the higher output terminal (2.11 V) of output terminals A and C, data can be transmitted efficiently at a high-speed of 400 MHz or greater in the form of a small amplitude transmission of 220 mV. If the supply potential VCC1 of the driver circuit 101 is sufficiently higher than the potential of the higher output terminal (the potential corresponding to Vd of the driver circuit 101 in FIG. 11 is 2.11 V), a current can be applied from a PMOS transistor 1101 in a driver circuit 101 (as shown in FIG. 11) to the output terminal A or C. Therefore, data can be transmitted efficiently at a high-speed of 400 MHz or greater in the form of a small amplitude transmission of 220 mV, as mentioned above.
However, in the case where the difference between the supply voltage VCC1 of the transmitting device 120 and the supply voltage VCC2 of the receiving device 130, and the difference between the ground voltage GND1 of the transmitting device 120 and the ground voltage GND2 of the receiving device 130 are relatively large, the potentials of the output terminals A and C of the driver circuit 101 of the transmitting device 120 (i.e., the potential of the transmission paths 110) may become infinitely close to the supply voltage VCC1 of the driver circuit 101, or even higher than the supply voltage VCC1 of the driver circuit 101, thereby making it difficult or impossible to apply a current from the driver circuit 101 to the transmission path 110. In other words, such a state causes a problem of not being able to transmit data.
FIG. 9B illustrates the problem caused by the difference between the ground potential GND1 of the transmitting device 120 and the ground potential GND2 of the receiving device 130 in the signal transmitting/receiving circuit 1000 shown in FIG. 9A. FIG. 10B illustrates the problem caused by the difference between a supply voltage VCC1 of a transmitting device 220 and a supply voltage VCC2 of a receiving device 230 in a signal transmitting/receiving circuit 2000 as shown in FIG. 10A. These problems will now be more specifically described in reference to FIGS. 9A through 10B.
FIGS. 9A and 9B show the case where the ground potential GND1 of the transmitting device 120 and the ground potential GND2 of the receiving device 130 are different. More specifically, it is assumed that the ground potential GND2 of the receiving device 130 is higher than the ground potential GND1 of the transmitting device 120. In this case, as shown in FIG. 9B, if the intermediate potential Vcm of the pair of differential lines 103A and 103C becomes higher than the supply voltage VCC1 of the driver circuit 101 of the transmitting device 120, it is impossible to apply a current. This difference between the ground potentials (GND2−GND1) is prone to occur when data is transmitted/received between different appliances grounded at different sites. A typical example of this is the case where the transmitting device 120 is a floor model VCR whose power is supplied from an outlet. In such a case, the ground potential GND1 is determined by the ground potential of the outlet. If the corresponding receiving device 130 is a video camera operating on an internal battery, the ground of the video camera is only connected to the housing of the video camera. Therefore, the ground of the camera will be a ground potential GND2, which may inevitably be different from the ground potential of the outlet. In the case where the power is supplied from such a floor model VCR to such a video camera via a cable (esp. IEEE 1394 and the like), the ground potential GND2 of the video camera may become about 0.5 V to 1.0 V higher than the ground potential GND1 of the floor model VCR (i.e., GND2=GND1+0.5 V to 1.0 V) due to the cable resistance.
In this case, the intermediate potential Vcm generated by the receiving device 130 appears higher (e.g., 0.5 V to 1.0 V) than the ground potential GND1 of the transmitting device 120, with a general tendency as shown in FIG. 9B. For example, if the intermediate potential is set at 2.0 V in the receiving device 130, it will become 2.5 V to 3.0 V in the transmitting device 120. If the supply voltage VCC1 of the driver circuit 101 in the transmitting device 120 is set at 2.5 V, the potential Vd shown in FIG. 11 will be, for example, 2.61 V to 3.11 V, which means VCC1≦Vd. Therefore, a problem exists when the PMOS transistor 1101 shown in FIG. 11 is not able to apply a current to the output terminals A and C.
FIG. 10A shows the case where the supply voltage VCC1 of the transmitting device 220 and the supply voltage VCC2 of the receiving device 230 are different. More specifically, it is assumed that the supply voltage VCC2 of the receiving device 230 is higher than the supply voltage VCC1 of the transmitting device 220. In this case, as shown in FIG. 10B, the intermediate potential Vcm of the cable becomes higher than the supply voltage VCC1 of a driver circuit 201 in the transmitting device 220, so that it is impossible to apply a current.
In a transmitting/receiving apparatus used for a digital video disc apparatus and the like (where a signal processing LSI corresponds to the transmitting device 220 and a servomotor controlling IC corresponds to the receiving device 230), this difference between the supply voltages (VCC2−VCC1) is inevitable from the system designing point of view. The most crucial reason for this is as follows: with a view to reducing the cost and the mounting area, there is a trend for developing highly integrated single-chip transmitting devices for utilizing the most recent device technologies. This, in turn, is because a signal processing LSI in a transmitting device can be implemented as digital circuits, so that the signal processing LSI can be mounted on a single chip together with a variety of other digital processing LSIs. Therefore, as shown in FIG. 12, the CMOS devices' supply voltage has been reduced over generations, e.g., from 5.0 V to 3.0 V, 3.0 V to 2.5 V, 2.5 V to 1.8 V, and so on.
On the other hand, as to ICs for controlling a servomotor associated with a receiving device, their supply voltage has not been changed over generations, but rather has remained constant at 5.0 V. This is because such an IC is usually a bipolar device, which is an analog circuit formed of semiconductors for driving mechanical systems such as a servomotor. Moreover, since such an IC is seldom required to incorporate a new function in each product generation, its design is usually not changed for five years or so, once designed. Therefore, it is impractical to change the circuits in the receiving device. In view of such a trend, FIGS. 10A and 10B represent the case where the supply voltage VCC2 of the receiving device 230 is higher than the supply voltage VCC1 of the transmitting device 220.
If the receiving device 230 is designed so that the intermediate potential Vcm is ½ of the supply voltage, then Vcm will be 2.5 V=(5 V×½). Therefore, with reference to FIG. 10B, those skilled in the art will readily understand that the supply voltage VCC1 of the transmitting device 220 should be set lower than 3.3 V if the design rule is 0.25 μm or less in order to achieve a high integration. If the design of the receiving circuit is changed each time the design of the transmitting circuit is changed, this problem can of course be solved to some degree. It is, however, impractical to reduce the product life of the IC only for the sake of redesigning the intermediate potential Vcm when there is no need to incorporate a new function, since it causes a cost increase. Moreover, in the case where only a low supply voltage is available to the transmitting device, the value of the intermediate potential Vcm may have to be set at 1.0 V or less. In this case, the circuits in the receiving device require a drastic redesign since an intermediate potential Vcm has to be set at 1.0 V or lower with a supply voltage of 5 V. It is readily understood this causes cost increase and unstable operation problem.